A power generation method comprising driving a diesel engine and exhausting exhaust gas to the diesel engine, driving a power turbine by the exhaust gas exhausted from the diesel engine, generating steam by introducing the exhaust gas from the diesel engine to an exhaust-gas heat exchanger, and driving the steam turbine by the steam generated by the exhaust-gas heat exchanger. Wherein, when demand power decreases and generated power becomes surplus, supply of the exhaust gas to the power turbine is reduced, and, after output of the power turbine is reduced to a minimum, supply of the steam to the steam turbine is reduced.